ResourceManager trait impl

Introduces the DefaultResourceManager struct. The core of methods
that will be used to inform the HTLC forward decisions are
add/resolve_htlc.

- add_htlc: Based on resource availability and reputation, it
evaluates whehther to forward or fail the HTLC.

- resolve_htlc: Releases the bucket resources used from a HTLC
previously added and updates the channel's reputation based on HTLC
fees and resolution times.

Author: elnosh